1. Field of the Invention
The invention relates to hybrid system of a hybrid vehicle that uses an engine and a rotary machine as power sources.
2. Description of Related Art
One such known hybrid system includes an engine, two rotary machines (e.g., electric motors), and a power split device (i.e., a planetary gear unit). In this hybrid system, a rotating shaft of the engine, rotating shafts of a first rotary machine and a second rotary machine and driving wheels, are connected to corresponding rotating elements of the power split device. For example, in a hybrid system described in Japanese Patent Application Publication No. 2008-120234 (JP 2008-120234 A) below, a planetary gear unit capable of operating as a speed change unit is arranged between the engine and the power split device. This speed change unit includes a rotating element that is connected to the rotating shaft of the engine, and a rotating element that is connected to a rotating element of the power split device. The rotating element of the power split device is a rotating element other than a rotating element that is connected to the rotating shaft of the first rotary machine and the rotating shaft of the second rotary machine, and the driving wheels. Moreover, the hybrid system described in JP 2008-120234 A is provided with an engagement device (i.e., a switching device) that changes speed ratios of the speed change device by switching the peed change device between a differential rotation possible state and a differential rotation not possible state. A hybrid system described in Japanese Patent Application Publication No. 2008-120234 (JP 2008-120234 A) and Japanese Patent Application Publication No. 2008-265600 (JP 2008-265600 A) is provided with a clutch (i.e., a fixed element) that fixes a rotating shaft of the engine. With this hybrid system, the vehicle is able to travel using the power of both the first rotary machine and the second rotary machine, by engaging this clutch and stopping the rotation of the rotating shaft of the engine. Therefore, in this hybrid system, the power source to be used for traveling is selected by controlling the engine, the first rotary machine, the second rotary machine, and the clutch, according to the required driving force. Also, with the hybrid system in JP 2008-265600 A, when traveling by the power of both the first rotary machine and the second rotary machine, the torque split of the first rotary machine and the second rotary machine is determined according to the efficiency of both the first rotary machine and the second rotary machine.
With a vehicle that employs a layout in which the engine is longitudinal mounted, such as a FR (Front-engine, Rear-wheel drive) vehicle, a power transmitting apparatus such as a transmission is covered by a floor tunnel below a floor panel. A floor tunnel is a tunnel portion of the body floor panel. Therefore, when a hybrid system having a structure such as that described in JP 2008-120234 A is mounted in this type of vehicle, a first rotary machine, a second rotary machine, a speed change device, a power split device, and an engagement device and the like are all arranged beneath this floor tunnel. Here, typically the floor tunnel becomes smaller in diameter toward the vehicle rear side, matching the shape of the case of the transmission. Therefore, when mounting this kind of hybrid system, it is desirable that the first rotary machine which, from among these, has a large diameter, be arranged toward the front of the vehicle in the floor tunnel. However, with this hybrid system, a hydraulically actuated engagement device has to be arranged farther toward the vehicle rear than the first rotary machine due to the arrangement of this first rotary machine, so the arrangement of an oil passage to supply hydraulic pressure to this engagement device is difficult.